1. Field of the Invention
The present invention relates to an image forming apparatus of electrophotographic system such as copier, printer or facsimile apparatus having a function of forming an image on a sheet or other transfer material (recording medium).
2. Description of the Related Art
Conventionally, in a color image forming apparatus, photosensitive members are used as independent image bearing members for colors of yellow, magenta, cyan and black, and electrostatic latent images are formed by emitting laser light to photosensitive members, these electrostatic latent images are developed by toners of individual colors, obtained toner images are sequentially transferred onto an intermediate transfer belt and overlaid, and transferred in batch on a sheet of recording medium, and a color image is obtained, which is known as a tandem type color image forming apparatus.
In such tandem type color image forming apparatus, a scanning optical device for emitting laser light to photosensitive members comprises a light source for emitting laser light according to image information, a rotary polyhedral mirror for deflecting and scanning the laser light emitted from the light source, an f-theta lens for scanning the laser light deflected and scanned by the rotary polyhedral mirror at uniform speed and forming a spot image on photosensitive member, a folding mirror for reflecting the laser light in predetermined direction, and a beam detection sensor for receiving the laser light reflected by the rotary polyhedral mirror and generating a beam detection signal, which are incorporated in a casing, and the laser light is emitted to the photosensitive member from a slit opening in the casing.
In such scanning type optical device, in order to lower the cost and reduce the size, as exposure means of laser light to photosensitive drums, the rotary polyhedral mirror as deflecting and scanning means is shared by a plurality of light sources, and one rotary polyhedral mirror is used for deflecting and scanning laser lights from the plurality of light sources to expose by emitting to the plurality of photosensitive drums, and in this scanning type optical device, in order to reduce the size, the laser beam is folded within the casing of the scanning type optical device, and the photosensitive members and the scanning type optical device are disposed closely to each other.
Specifically, as disclosed in Japanese Patent Application Laid-Open No. 2004-21133 (patent document 1), in a scanning type optical device for deflecting and scanning laser lights from a plurality of light sources by one rotary polyhedral mirror simultaneously, and exposing by emitting to a plurality of photosensitive drums, the plurality of laser beams from the rotary polyhedral mirror are scanned in an array in remote-close direction to the photosensitive members, and the laser beam closer to the photosensitive member side out of plural laser beams is set away from the photosensitive member so as to cross with other laser beam, and then by emitting to the photosensitive member, the length of optical path is assured.
In the tandem type color image forming apparatus, further reduction of size is demanded, and so as not to increase the dimension in lateral width direction or vertical height direction in the occupied space of image forming unit incorporating independent photosensitive members of yellow, magenta, cyan and black colors, each photosensitive member is inclined to the installation plane of image forming apparatus, and it is designed to expose by the scanning type optical device from the lower side of the photosensitive members.
If attempted to reduce the size in the configuration of patent document 1, for effective utilization of space, limitation in height direction of optical unit differs between both sides of irradiation of laser from rotary polyhedral mirror. That is, since the distance is shorter between the photosensitive member and the mirror for folding the laser folded in a direction departing from the photosensitive member toward the photosensitive member, the position of folding mirror cannot be set sufficiently apart from the photosensitive member, and, as a result, sufficient length of optical path is not assured in such region.